Vibration sensors are commonly used for monitoring the vibrations of structures such as buildings and bridges; vehicles such as ships, airplanes, autos and trains; and tools and machinery in factories. Many applications—particularly high-precision tools and machinery—require real-time monitoring to detect any abnormal vibration, because excess vibration in a tool's or machine's environment can cause the tool or machine to malfunction, and certain vibrations originating from the tools or machines themselves serve as indications that maintenance is needed. Therefore, accurate in-line monitoring could significantly reduce the down time and associated cost.
Mechanical vibrations in a structure, vehicle, tool or machine can occur over a wide range of intensities and frequencies, depending on the details of construction and operation. Available state-of-the-art vibration sensors, however, have features that make them unsuitable for use over a wide range of applications. On one hand, highly sensitive vibration sensors with low noise floors (i.e., the minimum vibration intensity the sensor can detect) are large in size, making them difficult to use for applications requiring a small form factor and light weight, such as monitoring robotic arms. They are also prohibitively expensive for applications that require a large number of sensors, such as increasingly important large-scale sensor network applications. On the other hand, very small, lightweight sensors such as microelectromechanical (MEMS) based sensors are not sensitive enough at low frequency ranges.